Add contrast or saturation to render?
The compositor has a whole section of nodes for image control.

To use the compositor open the Node Editor window and set it to compositing mode:



There are nodes for brightness contrast and hue/saturation:



On top of that there are nodes for levels, color correction, curves and other ways to affect a rendered image.

Quoting from the wiki:

These nodes play with the colors in the image. They adjust the image's color intensity, adjust contrast and intensity, and, most importantly, mix two images together by color, transparency, or distance.



For an in depth explanation on each one of those nodes refer to Manual